Controlled preparation of Ni–Cu alloy catalyst via hydrotalcite-like precursor and its enhanced catalytic performance for methane decomposition

Abstract

Composition-uniform Ni–Cu/Al2O3 alloy catalysts have been prepared from Ni–Cu–Al hydrotalcite-like compounds (HTlcs) and tested for methane decomposition at 650°C. The catalysts were characterized by XRD, XPS, H2 chemisorption, H2-TPR, STEM-EDX, SEM, TEM, and Raman. The characterizations reveal that calcination of Ni–Cu–Al HTlcs leads to Ni(Cu,Al)O oxide solid solution, both nickel and copper ions being homogeneously distributed in HTlcs as well as in Ni(Cu,Al)O, and upon reduction the well-mixed Cu2+/Ni2+ species are step-wise reduced to form composition-uniform Ni–Cu alloy with an average size of 9.5–10.4 nm. Alloying Ni with an appropriate amount of Cu remarkably enhances the catalytic life and carbon yield. The highest carbon yield of 132.9 g-C/g-cat is obtained at atomic ratio of Ni:Cu = 7:3, which is about 78 times that of the Ni/Al2O3 counterpart. Moreover, carbon morphology is changed from thin CNTs to thick fishbone-CNFs and platelet-CNFs depending on the copper content. Under the reaction atmosphere, Ni–Cu alloy is sintered to large particles by contact with methane. It is suggested that Ni–Cu alloying favors the formation of large alloy particles, which inhibits methane dissociation and enhances carbon bulk diffusion, thus facilitating the CNFs growth and leading to a significant increase of carbon yield.